1. Field of the Invention
The invention relates generally to adhesive blends and composite structures made therefrom and, more particularly, the invention relates to adhesive blends useful in adhering to polyolefin, polar and polystyrene substrates useful in food packaging and related applications.
2. Description of Background Technology
There has long been a perceived need for laminate structures comprising one or more of a food contact or sealing layer, a barrier layer, and a structural layer for use in the packaging and storage of food. Food contact/sealing layers often comprise non-polar polyolefins such as low density polyethylene (LDPE), for example, while typical barrier layers comprise substrates of polymers such as ethylene vinyl alcohol (EVOH), for example. Structural layers may comprise styrene polymers, which may contain rubber. High impact polystyrene (HIPS) is especially attractive as a structural layer, given its attractive physical properties, ease of extrusion processing, thermoforming, and cutting.
Such composite structures are useful as semi-rigid food packaging (e.g. thermoformed cups) or cast or blown film useful for packaging meat or cheese. Another application is in the fabrication of refrigerator liners that must form a good barrier against hydrohalocarbon blowing agent penetration that can lead to loss of refrigerant and solvent induced stress loading of various polymers.
It is desirable to form such structures by coextrusion or another method wherein a tie layer of adhesive is sandwiched between adjacent, often diverse substrates. Given the diversity of substrates useful in a laminate structure, however, one problem in the past has been to provide an adhesive tie layer which gives acceptable adhesion to each of the substrates present in the structure while being easily processable (i.e., thermoformable, readily die cut, etc.) without permanently changing form after thermoforming. For example, some adhesives remains soft or stringy after thermoforming, compromising the ease of processing by die cutting.
Adhesives useful in this type of composite structure must also be approved by the U.S. Food and Drug Administration or the European Economic Community regulatory authority which approves such materials for use in food packaging.